1. Field of the Invention
The present invention relates to an apparatus for evaluating the deteriorated state of a hydrocarbon adsorbent that is disposed in the exhaust passage of an internal combustion engine.
2. Description of the Related Art
Some known systems for purifying exhaust gases emitted from internal combustion engines have an exhaust gas purifier disposed in the exhaust passage, which may comprise a hydrocarbon adsorbent such as zeolite or a hydrocarbon adsorbing catalyst comprising a composite combination of a hydrocarbon adsorbent and a three-way catalyst, for adsorbing hydrocarbons (HC) in the exhaust gas while the catalytic converter such as a three-way catalyst or the like is not functioning sufficiently, i.e., while the catalytic converter is not sufficiently activated as when the internal combustion engine starts to operate at a low temperature. The hydrocarbon adsorbent has a function to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100xc2x0 C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250xc2x0 C., for example.
It has been desired in the art to evaluate the deteriorated state of a hydrocarbon adsorbent incorporated in an exhaust gas purifier in order to determine when to replace the exhaust gas purifier. The applicant of the present application has proposed the following technique of evaluating the deteriorated state of a hydrocarbon adsorbent: The hydrocarbon adsorbent is capable of adsorbing not only hydrocarbons contained in exhaust gases, but also moisture contained in exhaust gases. The ability of the hydrocarbon adsorbent to adsorb moisture, i.e., the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent, is highly correlated to the ability of the hydrocarbon adsorbent to adsorb hydrocarbons, i.e., the maximum amount of hydrocarbons that can be adsorbed by the hydrocarbon adsorbent. As the hydrocarbon adsorbent progressively deteriorates, both the ability to adsorb moisture and the ability to adsorb hydrocarbons are progressively lowered in the same manner. Therefore, when the ability of the hydrocarbon adsorbent to adsorb moisture is evaluated, the ability of the hydrocarbon adsorbent to adsorb hydrocarbons can also be evaluated based on the evaluated ability to adsorb moisture. According to the technique proposed by the applicant, a humidity sensor is disposed downstream of the hydrocarbon adsorbent or humidity sensors are disposed respectively downstream and upstream of the hydrocarbon adsorbent. The deteriorated state of the hydrocarbon adsorbent is evaluated based on the transition of an output signal from the humidity sensor or sensors after the internal combustion engine has started to operate.
For adequately evaluating the deteriorated state of the hydrocarbon adsorbent using output data from the humidity sensor or sensors in various many environments that the internal combustion engine is subject to while in operation, the humidity sensor or sensors that are exposed to high-temperature exhaust gases are required to be highly durable over a long period of time, and also required to have minimum changes in the aging characteristics thereof and to suffer minimum characteristic variations among individual units of humidity sensors.
However, it is generally difficult for humidity sensors to fully satisfy all the above requirements. It would need a large expenditure of money and labor to develop humidity sensors that fully satisfy all the above requirements, and such humidity sensors would be highly expensive to manufacture.
It is therefore an object of the present invention to provide an apparatus for adequately evaluating the deteriorated state of a hydrocarbon adsorbent by compensating for changes in the characteristics of humidity sensors that are used and also compensating for characteristic variations among individual units of the humidity sensors.
Basic principles of the present invention will first be described below. The humidity which is present downstream of a hydrocarbon adsorbent disposed in an exhaust passage of an internal combustion engine after the internal combustion engine has started to operate, generally undergoes the following transition: Immediately after the internal combustion engine has started to operate, the humidity which is present downstream of the hydrocarbon adsorbent is of a relatively low level as moisture contained in the exhaust gas is adsorbed by the hydrocarbon adsorbent. When the adsorption of moisture in the exhaust gas by the hydrocarbon adsorbent is saturated, the humidity which is present downstream of the hydrocarbon adsorbent changes to a tendency to monotonously increase from a low humidity to a high humidity and is eventually converged to a substantially constant high humidity level. Therefore, a changing timing at which the humidity downstream of the hydrocarbon adsorbent changes to a tendency to monotonously increase from a low humidity to a high humidity after the internal combustion engine has started to operate corresponds to a timing at which the adsorption of moisture by the hydrocarbon adsorbent is saturated. Consequently, a total amount of moisture supplied to the hydrocarbon adsorbent up to the changing timing after the internal combustion engine has started to operate signifies a maximum quantity of moisture that can be adsorbed by the hydrocarbon adsorbent. Accordingly, if the total amount of moisture or a quantity corresponding thereto is recognized, then it is possible to evaluate the ability of the hydrocarbon adsorbent to adsorb moisture and hence the deteriorated state of the hydrocarbon adsorbent.
For evaluating the deteriorated state of the hydrocarbon adsorbent, it is necessary to detect the changing timing. The changing timing can be recognized based on an appropriate characteristic quantity representing a change in an output signal of a humidity sensor that is disposed downstream of the hydrocarbon adsorbent. For example, after the internal combustion engine has started to operate, the time when a change in the output signal of the humidity sensor from a low humidity level to a high humidity level exceeds a preset value can be detected as the changing timing. However, the transition of the output signal of the humidity sensor is affected by not only the deteriorated state of the hydrocarbon adsorbent, but also a characteristic change of the humidity sensor due to a deterioration thereof. As the humidity sensor is progressively deteriorated, the rate of change of the output signal of the humidity sensor is lowered when the humidity represented by the output signal of the humidity sensor changes to the tendency to monotonously increase. Therefore, providing the preset value for detecting the changing timing is constant, then the detected changing timing varies depending on the deteriorated state of the humidity sensor even if the deteriorated state of the hydrocarbon adsorbent remains constant, and hence the total quantity of moisture supplied to the hydrocarbon adsorbent up to the detected changing timing changes. Therefore, it is preferable to correct the preset value for detecting the changing timing depending on the characteristic change of the humidity sensor due to its deterioration, thereby compensating for the characteristic change of the humidity sensor.
According to a first aspect of the present invention, there is provided an apparatus for evaluating a deteriorated state of a hydrocarbon adsorbent disposed in an exhaust passage of an internal combustion engine for adsorbing hydrocarbons contained in an exhaust gas emitted from the internal combustion engine, based on the transition characteristics of an output signal of a humidity sensor which is disposed in the exhaust passage downstream of the hydrocarbon adsorbent after the internal combustion engine has started to operate, the apparatus comprising changing timing detecting means for detecting a changing timing at which a humidity represented by the output signal of the humidity sensor changes to a tendency to monotonously increase from a low humidity to a high humidity based on a comparison between a characteristic quantity of a change in the output signal of the humidity sensor and a preset value, after the internal combustion engine has started to operate, evaluating parameter acquiring means for acquiring data representing a total amount of moisture carried by the exhaust gas to the hydrocarbon adsorbent up to the changing timing after the internal combustion engine has started to operate, as a deterioration evaluating parameter for evaluating the deteriorated state of the hydrocarbon adsorbent, characteristic change detecting means for detecting a characteristic change of the humidity sensor based on the output signal of the humidity sensor under a predetermined condition, and characteristic change compensating means for correcting the preset value to detect the changing timing based on the characteristic change detected by the characteristic change detecting means.
In the first aspect of the present invention, the preset value for detecting the changing timing is corrected based on the characteristic change of the humidity sensor detected based on the output signal of the humidity sensor under the predetermined condition, e.g., a condition with respect to the timing to acquire the output signal of the humidity sensor, an operating condition of the internal combustion engine, etc. Thus, it is possible to appropriately detect the changing timing irrespective of the characteristic change of the humidity sensor. The data representing the total amount of moisture supplied to the hydrocarbon adsorbent up to the changing timing is acquired as the deterioration evaluating parameter for evaluating the hydrocarbon adsorbent. The deterioration evaluating parameter thus acquired corresponds to the total amount of moisture that can be adsorbed by the hydrocarbon adsorbent, and represents the deteriorated state (extent of deterioration) of the hydrocarbon adsorbent. Regardless of the characteristic change of the humidity sensor, the hydrocarbon adsorbent is more deteriorated as the value of the deterioration evaluating parameter is smaller. Thus, the deteriorated state of the hydrocarbon adsorbent can adequately be evaluated based on the value of the deterioration evaluating parameter. Since the characteristic change of the humidity sensor can thus be compensated for, requirements for the steadiness of the characteristics of the humidity sensor are made less stringent, allowing the cost required to develop the humidity sensor and the cost required to manufacture the humidity sensor to be reduced.
In the first aspect of the present invention, the characteristic change of the humidity sensor detected by the characteristic change detecting means represents, for example, a characteristic change of a brand-new humidity sensor from predetermined reference characteristics.
In the first aspect of the present invention, the characteristic quantity to be compared with the preset value may comprise a change in the output signal of the humidity sensor from a certain level to a high humidity level at the time the internal combustion engine starts operating, or a rate of change of the output signal of the humidity sensor, i.e., a change in the output signal of the humidity sensor per given time. Generally, however, the output signal of the humidity sensor prior to the changing timing suffers a slight variation due to a disturbance or the like. Therefore, it is preferable to eliminate such a variation in the output signal of the humidity sensor prior to the changing timing for the purpose of preventing the changing timing from being detected in error.
In the first aspect of the present invention, the changing timing detecting means should preferably comprise means for sequentially retrieving states in which the humidity represented by the output signal of the humidity sensor takes a minimum value, after the internal combustion engine has started to operate, and detecting the changing timing when a change in the output signal of the humidity sensor from the latest minimum value in the retrieved states to the high humidity, serving as the characteristic quantity, exceeds the preset value.
With the above arrangement, even if the output signal of the humidity sensor varies due to a disturbance or the like prior to the changing timing, it is possible to appropriately detect the changing timing at which the humidity represented by the output signal of the humidity sensor changes to the tendency to monotonously increase to the high humidity.
In the first aspect of the present invention, the characteristic change detecting means should preferably comprise means for determining a change, from a predetermined reference value, of the output signal of the humidity sensor which is of a substantially constant high humidity level, as characteristic change detecting data representative of the characteristic change of the humidity sensor, after the changing timing is detected by the changing timing detecting means, and detecting the characteristic change of the humidity sensor based on the characteristic change detecting data.
Specifically, as will be described in greater detail later on, when the adsorption of moisture by the hydrocarbon adsorbent is saturated, the humidity downstream of the hydrocarbon adsorbent monotonously increases from a low humidity to a high humidity and thereafter converges to a substantially constant high humidity, which is inherent in the exhaust gas. While the humidity at the location of the humidity sensor is being substantially constant, the level of the output signal of the humidity sensor changes as the humidity sensor is progressively deteriorated. According to the present invention, after the changing timing is detected by the changing timing detecting means, a change, from a predetermined reference value, of the output signal of the humidity sensor which is of a substantially constant high humidity level is determined as characteristic change detecting data representative of the characteristic change of the humidity sensor. Then, the characteristic change of the humidity sensor is detected based on the characteristic change detecting data. In this manner, the characteristic change of the humidity sensor can appropriately be detected. The reference value may be the value of an output signal which is generated by a brand-new humidity sensor at the above constant humidity. If the characteristic change detecting data is used to detect the characteristic change of the humidity sensor, since the characteristic change detecting data is determined after the changing timing is detected, the preset value is corrected based on the characteristic change of the humidity sensor that is detected based on the characteristic change detecting data when the internal combustion engine is operated next time or more subsequently.
In the first aspect of the present invention, the apparatus should preferably further comprise characteristic data holding means associated with the humidity sensor for holding, in advance, data with respect to characteristics of each individual unit of the humidity sensor, and the characteristic change detecting means comprises means for detecting the characteristic change of the humidity sensor based on the output signal of the humidity sensor under the predetermined condition and the data held by the characteristic data holding means.
With the above arrangement, since the characteristic change of the humidity sensor is detected using the data held by the characteristic data holding means associated with the humidity sensor, i.e., the data with respect to characteristics of each individual unit of the humidity sensor, when the preset value for detecting the changing timing is corrected, not only characteristic changes of the humidity sensor, but also characteristic variations among individual units of the humidity sensor can be compensated for. As a result, requirements for the steadiness of the characteristics of individual sensors are made less stringent, and hence the cost required to develop the humidity sensor and the cost required to manufacture the humidity sensor are reduced.
As described above, if a change from the reference value of the output signal of the humidity sensor which is of a substantially constant humidity level is used as the characteristic change detecting data after having detected the changing timing, then the apparatus has characteristic data holding means associated with the humidity sensor for holding, in advance, data for identifying the reference value with respect to the characteristic change detecting data as data with respect to characteristics of each individual unit of the humidity sensor, and the characteristic change detecting means comprises means for determining the characteristic change detecting data using the reference value which is identified by the data held by the characteristic data holding means.
With the above arrangement, the reference value which serves as a reference for detecting the characteristic change of the humidity sensor is adjusted to match the characteristics of each individual unit of the humidity sensor. As a consequence, the characteristic change of the humidity sensor can be detected while appropriately compensating for characteristic variations of individual units of the humidity sensor.
In the first aspect of the present invention wherein characteristic variations of individual units of the humidity sensor are compensated for, the characteristic data holding means should preferably comprise a resistive element having a resistance depending on the data with respect to characteristics of each individual unit of the humidity sensor.
The characteristic data holding means which comprises a resistive element can be manufactured inexpensively and have a simple structure. Inasmuch as the resistance of the resistive element can be detected relatively easily, the data with respect to the characteristics of each individual unit of the humidity sensor can easily be recognized. The characteristic data holding means should preferably be provided on a connector by which the humidity sensor is connected to an electronic circuit unit, for example, for processing the output signal of the humidity sensor.
In the first aspect of the present invention, the evaluating parameter acquiring means should preferably comprise means for generating integrated moisture quantity data representative of sequentially integrated data of a quantity of moisture contained in the exhaust gas emitted from the internal combustion engine from the start of operation of the internal combustion engine, and means for acquiring the integrated moisture quantity data at the changing timing as the deterioration evaluating parameter.
Specifically, the integrated moisture quantity data at the changing timing represents a total quantity of moisture that is supplied to the hydrocarbon adsorbent until the adsorption of moisture by the hydrocarbon adsorbent is saturated after the internal combustion engine has started to operate. Therefore, the integrated moisture quantity data at the changing timing can be obtained as an appropriate deterioration evaluating parameter representative of the ability of the hydrocarbon adsorbent to adsorb moisture and hence the ability of the hydrocarbon adsorbent to adsorb hydrocarbons.
While the internal combustion engine is being shut off, the hydrocarbon adsorbent adsorbs some moisture that is present therearound. Therefore, when the internal combustion engine starts to operate, the humidity in the vicinity of the hydrocarbon adsorbent, including a region upstream thereof, is relatively low. Even after the internal combustion engine has started to operate, there is a short time delay until the exhaust gas generated by the internal combustion engine, i.e., the exhaust gas that is relatively highly humid, reaches a position near the inlet of the hydrocarbon adsorbent. Consequently, the humidity upstream of the hydrocarbon adsorbent is low for a relatively short period of time immediately after the internal combustion engine has started to operate. Thereafter, when the highly humid exhaust gas reaches the position near the inlet of the hydrocarbon adsorbent, the humidity upstream of the hydrocarbon adsorbent exhibits a tendency to monotonously increase from a low humidity to a high humidity, and then converges to a substantially constant high humidity level. The timing at which the humidity upstream of the hydrocarbon adsorbent changes from a low humidity to a high humidity may vary depending on the structure of the exhaust system of the internal combustion engine and the absorption of moisture by a catalytic converter that is disposed upstream of the hydrocarbon adsorbent. In this case, the timing at which the hydrocarbon adsorbent essentially starts adsorbing moisture in the exhaust gas in the exhaust gas tends to vary. For more adequately evaluating the deteriorated state of the hydrocarbon adsorbent, i.e., its ability to adsorb hydrocarbons and moisture, it is preferable that the changing timing at which the humidity upstream of the hydrocarbon adsorbent exhibits a tendency to monotonously increase from a low humidity to a high humidity be detected using a humidity sensor disposed upstream of the hydrocarbon adsorbent, and data representative of a total quantity of moisture that is supplied to the hydrocarbon adsorbent from the changing timing of the output signal from the upstream humidity sensor to the changing timing of the output signal from the downstream humidity sensor be acquired as a deterioration evaluating parameter. If the humidity sensor is provided not only downstream of the hydrocarbon adsorbent but also upstream of the hydrocarbon adsorbent, then it is preferable to compensate for both characteristic changes of the downstream humidity sensor and characteristic changes of the upstream humidity sensor.
According to a second aspect of the present invention, there is provided an apparatus for evaluating a deteriorated state of a hydrocarbon adsorbent disposed in an exhaust passage of an internal combustion engine for adsorbing hydrocarbons contained in an exhaust gas emitted from the internal combustion engine, based on the transition characteristics of output signals of upstream and downstream humidity sensors which are disposed in the exhaust passage respectively upstream and downstream of the hydrocarbon adsorbent after the internal combustion engine has started to operate, the apparatus comprising upstream changing timing detecting means for detecting a changing timing at which a humidity represented by the output signal of the upstream humidity sensor changes to a tendency to monotonously increase from a low humidity to a high humidity based on a comparison between a characteristic quantity of a change in the output signal of the upstream humidity sensor and a first preset value, after the internal combustion engine has started to operate, downstream changing timing detecting means for detecting a changing timing at which a humidity represented by the output signal of the downstream humidity sensor changes to a tendency to monotonously increase from a low humidity to a high humidity based on a comparison between a characteristic quantity of a change in the output signal of the downstream humidity sensor and a second preset value, evaluating parameter acquiring means for acquiring data representing a total amount of moisture carried by the exhaust gas to the hydrocarbon adsorbent from the upstream changing timing to the downstream changing timing, as a deterioration evaluating parameter for evaluating the deteriorated state of the hydrocarbon adsorbent, characteristic change detecting means for detecting a characteristic change of each of the humidity sensors based on the output signals of the humidity sensors under a predetermined condition, and characteristic change compensating means for correcting the first preset value and the second preset value based on the characteristic changes of the upstream humidity sensor and the downstream humidity sensor detected by the characteristic change detecting means.
In the second aspect of the present invention, the second preset value for detecting the changing timing with respect to the downstream humidity sensor is corrected based on the detected characteristic change of the downstream humidity sensor as with the first aspect of the present invention. The first preset value for detecting the changing timing with respect to the upstream humidity sensor is corrected based on the detected characteristic change of the upstream humidity sensor in the same manner as the second preset value with respect to the downstream humidity sensor. Therefore, the changing timings with respect to the upstream and downstream humidity sensors can appropriately be detected irrespective of the characteristic changes of those humidity sensors. The data representing the total amount of moisture supplied to the hydrocarbon adsorbent from the upstream changing timing to the downstream changing timing is acquired as the deterioration evaluating parameter, and the deterioration evaluating parameter thus acquired corresponds to the total quantity of moisture that can be adsorbed by the hydrocarbon adsorbent independently of characteristic changes of the humidity sensors and variations of the changing timing of the upstream humidity, and thus represents the deteriorated state (extent of deterioration) of the hydrocarbon adsorbent. Specifically, as the value of the deterioration evaluation parameter is smaller, the hydrocarbon adsorbent is more deteriorated independently of characteristic changes of the humidity sensors and variations of the changing timing of the upstream humidity. The deteriorated state of the hydrocarbon adsorbent can be evaluated more adequately based on the value of the deterioration evaluating parameter. Because the characteristic changes of both the humidity sensors can be compensated for, requirements for the steadiness of the characteristics of the humidity sensors are made less stringent, allowing the humidity sensors to have some characteristic changes. Consequently, the cost required to develop the humidity sensor and the cost required to manufacture the humidity sensor can be reduced.
In the second aspect of the present invention, the characteristic quantities with respect to both the upstream and downstream humidity sensors should preferably be the same as the characteristic quantity in the first aspect of the present invention. Specifically, the upstream changing timing detecting means should preferably comprise means for sequentially retrieving states in which the humidity represented by the output signal of the upstream humidity sensor takes a minimum value, after the internal combustion engine has started to operate, and detecting the changing timing with respect to the upstream humidity sensor when a change in the output signal of the upstream humidity sensor from the latest minimum value in the retrieved states to the high humidity, serving as the characteristic quantity with respect to the upstream humidity sensor, exceeds the first preset value. Similarly, the downstream changing timing detecting means should preferably comprise means for sequentially retrieving states in which the humidity represented by the output signal of the downstream humidity sensor takes a minimum value, after the internal combustion engine has started to operate, and detecting the changing timing with respect to the downstream humidity sensor when a change in the output signal of the downstream humidity sensor from the latest minimum value in the retrieved states to the high humidity, serving as the characteristic quantity with respect to the downstream humidity sensor, exceeds the second preset value.
With the above arrangement, the changing timings of the humidities detected by the upstream and downstream humidity sensors can adequately be detected even if the output signals of the humidity sensors are varied due to a disturbance or the like prior to the respective changing timings.
In the second aspect of the present invention, the characteristic change detecting means should preferably comprise means for determining a change, from a predetermined reference value, of the output signal of the upstream humidity sensor which is of a substantially constant high humidity level, as characteristic change detecting data representative of the characteristic change of the upstream humidity sensor, after the changing timing is detected by the upstream changing timing detecting means, determining a change, from a predetermined reference value, of the output signal of the downstream humidity sensor which is of a substantially constant high humidity level, as characteristic change detecting data representative of the characteristic change of the downstream humidity sensor, after the changing timing is detected by the downstream changing timing detecting means, and detecting the characteristic change of each of the humidity sensors based on the characteristic change detecting data with respect to each of the humidity sensors.
As described above in the first aspect of the present invention, the humidity at the location of the downstream humidity sensor finally converges to a substantially constant high humidity level after the changing timing of the downstream humidity, and the level of the output signal of the downstream humidity sensor at the substantially constant high humidity level changes as the downstream humidity sensor is progressively deteriorated. This also holds true for the upstream humidity sensor. Consequently, the characteristic change detecting data with respect to the humidity sensors are determined in the same manner as with the first aspect of the present invention, and the characteristic changes of the humidity sensors are detected based on the determined characteristic change detecting data with respect to the humidity sensors, so that the characteristic changes can appropriately be detected with respect to the respective humidity sensors.
The reference value with respect to each of the humidity sensors may be the value of an output signal which is generated by a brand-new humidity sensor at the above constant humidity. If the characteristic change detecting data is used to detect the characteristic change of each of the humidity sensors, since the characteristic change detecting data for each of the humidity sensors is determined after the changing timing with respect to each of the humidity sensors is detected, the first and second preset values are corrected based on the characteristic changes of the humidity sensors that are detected based on the characteristic change detecting data when the internal combustion engine is operated next time or more subsequently.
In the second aspect of the present invention, the apparatus should preferably further comprise characteristic data holding means associated respectively with the upstream and downstream humidity sensors, for holding, in advance, data with respect to characteristics of each individual unit of the upstream and downstream humidity sensors, and the characteristic change detecting means comprises means for detecting the characteristic changes of upstream and downstream humidity sensors based on the output signals of the upstream and downstream humidity sensors under the predetermined condition and the data held by the characteristic data holding means.
With the above arrangement, since the characteristic changes of the humidity sensors are detected using the data held by the characteristic data holding means associated respectively with the upstream and downstream humidity sensors, i.e., the data relative to the characteristics of the individual units of the upstream and downstream humidity sensors, when the first and second preset values for detecting the upstream and downstream changing timings are corrected, it is possible to compensate for not only characteristic changes of the humidity sensors, but also characteristic variations among individual units of the humidity sensors. As a result, requirements for the steadiness of the characteristics of individual sensors are made less stringent, and hence the cost required to develop the humidity sensors and the cost required to manufacture the humidity sensors are reduced.
For detecting the characteristic changes of the humidity sensors using the characteristic change detecting data, the apparatus should preferably further comprise characteristic data holding means associated respectively with the humidity sensors, for holding, in advance, data for identifying the reference values with respect to the characteristic change detecting data with respect to the respective humidity sensors as data with respect to characteristics of each individual unit of the humidity sensors, and the characteristic change detecting means comprises means for determining the characteristic change detecting data with respect to the respective humidity sensors using the reference values which are identified by the data held by the characteristic data holding means of the respective humidity sensors.
With the above arrangement, the reference values which serve as a reference for detecting the characteristic changes of the humidity sensors are adjusted to match the characteristics of individual units of the humidity sensors. As a consequence, the characteristic changes of the humidity sensors can be detected while appropriately compensating for characteristic variations of individual units of the upstream and downstream humidity sensors.
In the second aspect of the present invention wherein characteristic variations of individual units of the upstream and downstream humidity sensors are compensated for, as with the first aspect of the present invention, the characteristic data holding means should preferably comprise respective resistive elements having respective resistances depending on the data with respect to characteristics of individual units of the humidity sensors. The characteristic data holding means which comprise resistive elements can be manufactured inexpensively and have a simple structure. Inasmuch as the resistances of the resistive elements can be detected relatively easily, the data with respect to the characteristics of individual units of the humidity sensors can easily be recognized.
In the second aspect of the present invention, the evaluating parameter acquiring means should preferably comprise means for generating integrated moisture quantity data representative of sequentially integrated data of a quantity of moisture contained in the exhaust gas emitted from the internal combustion engine from the start of operation of the internal combustion engine, and means for acquiring the difference between the integrated moisture quantity data at the changing timing with respect to the downstream humidity sensor and the integrated moisture quantity data at the changing timing with respect to the upstream humidity sensor, as the deterioration evaluating parameter.
Specifically, the difference between the integrated moisture quantity data at the downstream changing timing and the integrated moisture quantity data at the upstream changing timing represents a total quantity of moisture that is supplied to the hydrocarbon adsorbent from the upstream changing timing to the downstream changing timing. Therefore, the difference obtained as the deterioration evaluating parameter is appropriate as representing the ability of the hydrocarbon adsorbent to adsorb moisture and hence the ability of the hydrocarbon adsorbent to adsorb hydrocarbons regardless variations of the upstream changing timing.
In either the first aspect or the second aspect of the present invention, the integrated moisture quantity data may be the data of an integrated moisture quantity itself, but may basically be any data insofar as it is substantially proportional to the integrated moisture quantity. For example, an integrated value of a quantity of fuel supplied to the internal combustion engine or an integrated value of a quantity of air drawn into the internal combustion engine may be used as the integrated moisture quantity data. If the internal combustion engine operates in a substantially constant mode such as an idling mode after it has started operating, then a period of time that has elapsed after the internal combustion engine has started operating may be used as the integrated moisture quantity data.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.